1. Field of the Invention
The present invention relates to an impedance matching device for matching the impedance of a high-frequency (radio-frequency) generator with the impedance of a load device.
2. Description of Related Art
If the impedance of the high-frequency generator is not matched with the impedance of the load device while supplying a high-frequency power from the high-frequency generator into the load device, the supply efficiency of the high-frequency power from high-frequency generator to the load device is low.
The impedance value of the high-frequency generator is a fixed value, e.g., 50 (.OMEGA.) or 75 (.OMEGA.). On the other hand, the impedance value of the load device not only varies widely depending on the structure thereof and its installation condition, but varies widely for a short time depending on a physical change caused inside by the high-frequency power applied to the load device, e.g., a temperature rise and the like, and also depending on a chemical change, e.g., generation of a gas and the like, caused by decomposition of a part of the raw material of the components in the load device.
In order to match the impedance of the high-frequency generator with the impedance of the load device, an impedance matching device is generally provided between the high-frequency generator and the load device.
Next, an earlier developed impedance matching device provided between the high-frequency generator and the load device will be explained with reference to FIGS. 12A to 12H.
FIGS. 12A to 12H are views showing circuit constructions of the main part of the earlier developed impedance matching device.
Although FIGS. 12A to 12H show a plurality of circuit constructions, because the fundamental circuit functions of the constructions are substantially the same, only the construction shown in FIG. 12A will be explained and the explanation for FIGS. 12B to 12H will be omitted.
Each of the impedance matching device shown in FIGS. 12A to 12H comprises a variable capacitance element VC and a variable inductance element VL.
An end of the variable capacitance element VC is connected to a terminal "a" and the other end thereof is connected to terminals "b" and "d". An end of the variable inductance element VL is connected to terminals "a" and an end of the variable capacitance element VC, and the other end of the variable inductance element VL is connected to a terminal "c".
For example, between the terminals "a" and "b", a high-frequency generator which is not shown is connected; and the terminals "c" and "d", a load device which is not shown is connected.
In order to match the impedance of the high-frequency generator with the impedance of the load device, the capacitance value of the variable capacitance element VC and the inductance value of the variable inductance element VL are changed.
Although a detailed explanation for the method is omitted, the standing wave between the terminals "a" and "b" is measured, and a voltage standing wave ratio circuit is operated on the basis of the measurement result. Thereafter, the capacitance value of the variable capacitance element VC, and the inductance value of the variable inductance element VL are computed to match the impedance of the high-frequency generator with the impedance of the load device. According to the computation result, the capacitance value of the variable capacitance element VC and the inductance value of the variable inductance element VL are changed.
However, in the above-described earlier developed impedance matching device, because the high-frequency power for industrial use is not less than several tens of watts, a variable capacitor is used as the variable capacitance element VC, and the capacitance value of the variable capacitor is changed by using a motor. The inductance value of a coil which is used as the variable inductance element VL is changed by moving a contact (slider) on the coil by using a motor.
According to the above-described method in which the capacitance value of the variable capacitor used as the variable capacitance element VC and the inductance value of a coil used as the variable inductance element VL are changed by using a motor, a great amount of time is necessary to match the impedance of the high-frequency generator with the impedance of the load device because the speed available to change the capacitance value and the inductance value is restricted by the operating speed of the motor.
There is another problem that when the impedance of the load device has suddenly changed after matching of the impedance of the high-frequency generator and the impedance of the load device was carried out, changing of the capacitance value of the variable capacitor used as the variable capacitance element VC and the inductance value of a coil used as the variable inductance element VL cannot follow the sudden change of the impedance of the load device.
There is a further problem that maintenance operations, e.g., a routine lubrication, a replacement of consumable parts and the like, are required.